The invention relates to an apparatus and a method in the treatment of the stock passed to a headbox of a paper machine or equivalent.
Centrifugal cleaning is needed in paper machines for separation of sand and contaminants. With today's technology, the cleaning efficiency of centrifugal cleaning deteriorates when the fiber consistency of the pulp suspension exceeds 1 percent. This limits the increasing of the feed consistency of the stock to be fed to the headbox. In practice, the slotted screen technique has made it unnecessary to use centrifugal cleaning for separating reject fibers, such as shives. A hydrocyclone plant is placed in the short circulation of the paper machine, where the flow rates are high, as high as 2000 l/s. To be operative, centrifugal cleaning requires a pressure difference of 120-150 kPa. In that connection, all (about 5) steps of the hydrocyclone plant require pumps, which represent as much as about 25 percent of the energy consumption of the short circulation. At a flow rate of 2000 l/s, the power consumption of centrifugal cleaning is about 1200 kW. A typical amount of fiber reject from centrifugal cleaning is about 0.1-0.2 percent of production. The loss of the filler pigments coming with coated broke is at its worst about 0.5 percent of machine production.
A filler recovery system is often incorporated in connection with the centrifugal cleaning of the short circulation. In addition to filler, the system must also process other rejects, such as fiber reject and sand, coming from the short circulation. In that case, the efficiency of the filler recovery system is not best possible.
Concepts are known in which the cleaning of the stock has been transferred from the short circulation to pulp lines. The consistency (about 3 percent) of the broke system is, however, not suitable for separation of sand with hydrocyclones.
When centrifugal cleaning is in the pulp line (e.g. chemical pulp, DIP or TMP), these pulps need not be cleaned again any more, but the debris, sand and non-disintegrating coating sheets of paper coming to the broke system via pulpers should be treated by means of hydrocyclones.